In recent years, the utilization of the magnetic loss of a magnetic material has attracted attention for the reduction of spurious radiation noise radiated from devices such as mobile phones and mobile computers operative at frequencies in a high frequency band of not lower than gigahertz.
The mechanism of the absorption of electromagnetic waves (the attenuation of spurious radiation noise) utilizing the magnetic loss of the magnetic material varies depending on a positional relationship between a noise source and the magnetic material. It is known that the generation of a high frequency electric current is suppressed by providing the magnetic material in close proximity to a noise transmission path.
It is also known that the magnitude of an equivalent resistive component occurring in this case depends on the magnitude of a magnetic loss term μ″, which is the imaginary part of the complex permeability μ=μ′−jμ″ of the magnetic material and is generally proportional to the magnitude of the magnetic loss term μ″ when the magnetic material has a constant surface area.
One exemplary electromagnetic wave absorber to be used for attenuating the spurious radiation noise by utilizing the aforesaid function of the magnetic material is a composite material prepared by dispersing fine magnetic particles in a binder such as a resin and formed into a predetermined shape such as a sheet shape.
Exemplary electromagnetic wave absorbers composed of such a composite material are disclosed in Japanese Unexamined Patent Publication No. 2001-200305 (Document 1), Japanese Unexamined Patent Publication No. 2002-158484 (Document 2), and “Advanced Technology and Applications of Novel Radio Wave Absorbers” edited by Osamu Hashimoto and published by CMC Co. on Mar. 1, 1999, page 134, lines 6-8, FIG. 1g (Document 3).
However, composite materials currently in practical use provide a small magnetic loss term μ″ on the order of about 5 to 6 in a high frequency band of not lower than gigahertz. Even if an electromagnetic wave absorber sheet having a thickness of about 1 mm is formed of any of these composite materials, the electromagnetic wave absorber sheet is merely capable of shielding about 50 to 60 percent of electromagnetic waves.
With a recent increasing trend toward the size reduction of the devices, the electromagnetic wave absorbers tend to suffer from a space limitation. Hence, there is a demand for an electromagnetic wave absorber which has a much smaller thickness and provides a greater magnetic loss term μ″ in a high frequency band of not lower than gigahertz than the conventional electromagnetic wave absorbers.
To meet the demand, an attempt has been made to increase the content of the magnetic particles in the composite material for improvement of the electromagnetic wave absorbing characteristic.
Further, the inventor of the present invention has contemplated to form an electromagnetic wave absorbing layer of the composite material on a surface of a printed wiring board constituting an internal circuit of any of the aforesaid devices for space saving by thickness reduction.
With a recent increasing trend toward higher output capacities of the devices, there is a requirement for further powerfully absorbing electromagnetic waves in a predetermined frequency range. However, the electromagnetic wave absorbers composed of the conventional composite materials cannot satisfy the requirement.
As the content of the magnetic particles is increased for the improvement of the electromagnetic wave absorbing characteristic, the content of the binder is relatively reduced, and the physical strength and formability of the electromagnetic wave absorber are correspondingly reduced. Therefore, the improvement of the electromagnetic wave absorbing characteristic has limitation.
This is also true for the electromagnetic wave absorbing layer formed on the surface of the printed wiring board. If the content of the binder is too low, the film formability is reduced, making it impossible to form a thin and uniform electromagnetic wave absorbing layer. Further, the strength of the film is reduced after the formation of the film. Therefore, the content of the magnetic particles cannot extremely be increased, so that the improvement of the electromagnetic wave absorbing characteristic has limitation.